Page 353 - Fiber Optic Communications Fund

334 Fiber Optic Communications

7.6 For a long-haul fiber-optic system consisting of 10 identical amplifiers with gain G = 30 dB and noise
figure F = 4.5 dB, it is desirable to have a OSNR of 15 dB (in 0.1 nm bandwidth) at the receiver. Find
n
the transmitter power launched to the fiber.
(Ans: −1.5 dBm.)

7.7 A 20,000-km transmission system at 10 Gb/s based on OOK with direct detection needs to be designed.
The required Q-factor at the receiver should be ≥ 5. Fiber loss = 0.18 dB/km and the loss is exactly
compensated by periodically placed amplifiers. Write a program to find the maximum amplifier spacing
allowed. Assume B = 7.5 GHz, R = 1 A/W. Ignore shot noise and thermal noise.
e
7.8 Explain how a larger amplifier spacing deteriorates the system performance in a long-haul fiber-optic
system.

7.9 For a cascaded chain of k amplifiers, show that
F n,2 − 1 F n,3 − 1 F n,k − 1
F n,eq = F n,1 + + +···+ , (7.175)
G G G G G ···G
1 1 2 1 2 k−1
where F n,j and G are the noise figure and gain of the jth amplifier, respectively.
j
7.10 Write a program to calculate the OSNR and BER of a long-haul fiber-optic system with
direct/coherent detection. Include shot noise, thermal noise, signal–spontaneous beat noise and
spontaneous–spontaneous beat noise (if applicable). Compare the BERs obtained using the
exact Q-factor and approximate Q-factor obtained by ignoring shot noise, thermal noise, and
spontaneous–spontaneous beat noise.

Further Reading

G. Keiser, Optical Fiber Communications, 4th edn. McGraw-Hill, New York, 2011.
G.P. Agrawal, Fiber-optic Communication Systems. John Wiley & Sons, Hoboken, NJ 2010.
K.P. Ho, Phase Modulated Optical Communication Systems. Springer-Verlag, Berlin, 2005.
J. M. Senior, Optical Fiber Communications, 2nd edn. Prentice-Hall, London, 1992.
S. Betti, G. DeMarchis, and E. Iannone, Coherent Optical Communication Systems. John Wiley & Sons, New York, 1995.
T. Okoshi and K. Kikuchi, Coherent Optical Fiber Communications. Kluwer Academic, Dordrecht, 1998.

References
[1] K.P. Ho, Phase Modulated Optical Communication Systems. Springer-Verlag, Berlin, 2005.
[2] S. Betti, G. De Marchis, and E. Iannone, Coherent Optical Communication Systems. John Wiley, Hoboken, NJ, 1995.
[3] K. Kikuchi and S. Tsukamoto, J. Lightwave Technol., vol. 26, p. 1817, 2008.
[4] G.P. Agrawal, Fiber-optic Communication Systems. John Wiley & Sons, Hoboken, NJ, 2010.
[5] C.R. Giles and E. Desurvire, J. Lightwave Technol., vol. 9, p. 147, 1991.
[6] Y. Yamamoto and T. Mukai, Opt. Quant. Electron., vol. 21, p. S1, 1989.
[7] P.A. Humblet and M. Azizoglu, J. Lightwave Technol., vol. 9, p. 1576, 1991.
[8] D. Marcuse, J. Lightwave Technol., vol. 8, p. 1816, 1990.
[9] D. Marcuse, J. Lightwave Technol., vol. 9, p. 505, 1991.